import java.util.Scanner;

public class Main {
    /**
     *
     * @param args
     */
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        int[] a=new int[n];
        long sum=0;
        for(int i=0;i<n;i++){
            a[i]=sc.nextInt();
            sum+=a[i];
        }
        long ret=0;
        for(int i=0;i<n;i++){
            ret+=a[i]*(sum-a[i]);
            sum-=a[i];
        }
        System.out.println(ret);
    }
}